Electricity meters are meters that, among other things, measure electrical energy flowing to a load, or at least through a conductor that feeds one or more loads. Electricity meters are often located at domestic residences for the purpose of measuring consumption of electricity by a residence for billing purposes. Electricity meters are also located on larger commercial and industrial structures for similar reasons. Electricity meters are often used for additional purposes, such as tracking energy usage trends, and/or load control.
With respect to load control, electricity meters sometimes include disconnect switches that automatically disconnect and reconnect a load to the utility power lines. Disconnect switches can be used for prepaid electricity services, as well as for load shedding. In prepaid electrical service situations, the disconnect switch automatically disconnects the load from the power lines once the customer has consumed the prepaid amount of energy. When additional energy is purchased, the disconnect switch reconnects the load to the power line. Because disconnect switches connect an entire customer load (such as a residence) to the power lines, the disconnect switch must be able to handle a significant amount of current, such as, for example 200 amperes. Mechanical switches are well-suited for switching currents of this magnitude.
In order to actuate the mechanical switches, an actuator such as a solenoid or motor typically must be used. In one example, the meter assembly includes an actuator in the form of a small motor that actuates the disconnect switch. Linear power supplies have been used to provide the motor driver circuit with power.
A problem has arisen with a residential electricity meter employing a linear power supply and a motor driver circuit. In particular, in one example, the motor driver circuit includes a capacitor that is discharged through a small motor to open or close the 200 ampere switch. The capacitor is then allowed to charge to the maximum level before a subsequent operation is performed. The capacitor provides a reserve of energy that is used to reduce the stress on the linear power supply during the relatively infrequent operation of the switch.
Charging the capacitor to the level of the power supply unregulated voltage Vur creates a problem in that under conditions of heavily loading the power supply with optional circuitry and under conditions of low line voltage the capacitor is charged to a voltage level insufficient to reliability operate the motor. To overcome this problem a transformer with a higher secondary voltage could be used. However, to maintain proper regulation, the capacity of the transformer would have to be increased proportionally. This would result in a physically larger transformer and higher cost. In general, electricity meters do not include excessive space to accommodate larger components, and cost is always a concern. Moreover, using a larger transformer would result in higher losses which reduces efficiency and increases internal heating.
One potential solution involves the use of a switched-mode supply. Use of switched-mode technology in place of a linear power supply is undesirable due to the increased complexity and cost and the potential for reduced reliability. Moreover, use of a higher secondary voltage transformer in conjunction with a switch mode DC regulator is also considered undesirable for reasons of complexity, cost, and the impact of accommodating the higher electrical noise associated with switched-mode typology.
There is a need, therefore, for low cost means to provide a larger voltage for the purposes of charging a motor drive circuit.